1. Field of Invention
The invention relates to an emission control apparatus of an internal combustion engine and, more particularly, to an emission control apparatus that has a plurality of emission control catalysts in an exhaust system of an internal combustion engine, and an emission control method thereof.
2. Description of Related Art
An example of the emission control apparatus having a plurality of emission control catalysts in an exhaust system of an internal combustion engine is disclosed in, for example, Japanese Patent Gazette No. 2727906.
According to this example, split passageways are provided in the exhaust system of the internal combustion engine, and each split passageway is provided with a filter loaded with a NOx absorbent that is a catalytic substance serving as an emission control catalyst. Provided upstream of each filter are a shutter valve, a fuel injection nozzle, and a secondary air introducing device.
During operation of the internal combustion engine, the shutter valves provided in the split passageways are alternately opened and closed, so that two filters are alternately used, that is, a first one of the filters is put into a resting mode when a second filter is used to absorb or trap nitrogen oxides (NOx) and particulates (e.g., soot) from exhaust gas, and the second filter is put into the resting mode when the first filter is in use.
In each split passageway, the fuel injection nozzle and the secondary air introducing device are disposed immediately upstream of the filter and downstream of the shutter valve. During closure of the shutter valve (the rest period of the filter), the filter is regenerated by supplying fuel and air to the filter to ignite and burn particulates on the filter which may cause clogging of the filter. That is, the fuel injection nozzles and the secondary air introducing devices are devices for recovering the emission control capability of the filters.
Thus, by alternately using two filters so that a first filter is regenerated while the second filter is in use, and the second filter is regenerated while the first filter is in use, the related-art emission control apparatus achieves both prevention of engine output reduction caused by the simultaneous clogging of the two filters and continuous emission control during operation of the engine.
A recent focus of attention is a generally termed “continuous regeneration” particulate filter that oxides and burns trapped particulates continuously without producing luminous flames.
This particulate filter has an emission control effect of removing particulates, such as soot and the like, by the chemical action of the catalytic substance, as well as the capability for removing nitrogen oxides (NOx), and is therefore drawing attention as an emission control catalytic device for diesel engines and the like. More specifically, this filter device incorporates a filter loaded with an active oxygen releasing agent as a catalytic substance, and controls (removes) exhaust emissions by oxidizing and burning particulates trapped on the filter via active oxygen.
In this filter device, therefore, the clogging with particulate deposits does not occur, and the switching use of filters as in the aforementioned related-art emission control apparatus is not necessary. Thus, the filter device allows good emission control using a single particulate filter.
In the aforementioned various emission control catalytic devices, the management of a catalyst bed temperature is critical since emission control is performed by action of the catalytic substances. That is, the exhaust gas purification rate (emission purification rate) changes in accordance with the activity of the supported catalytic substance. Therefore, in order to achieve a high purification rate, the catalyst bed temperature needs to be kept within an appropriate temperature range (generally termed “emission purification window”) as indicated in a purification rate diagram of FIG. 12.
With regard to an emission control catalytic device loaded with an active oxygen releasing agent or a NOx absorbent as mentioned above, the catalyst bed temperature range where high purification rate is achieved is relatively narrow. Therefore, for example, during long-hour idling operation or continuous high-load engine operation, etc., the bed temperature of the catalytic device excessively drops or rises, and often deviates from the aforementioned catalyst bed temperature range of high purification rate.
In a case where there is no effect of external thermal energy on the emission control catalyst, the catalyst bed temperature rises if high-temperature exhaust gas is supplied, and the catalyst bed temperature drops if low-temperature exhaust gas is supplied. That is, management of the catalyst bed temperature can be realized by adjustment of the temperature of exhaust gas, and high purification rate can be achieved by feeding exhaust gas at an appropriate temperature.
FIG. 13 is a diagram indicating a correlation between the temperature of an emission control catalyst and the exhaust gas temperature. As can be understood from this diagram, the catalyst bed temperature remains within the emission purification window while the temperature of exhaust gas flowing into the emission control catalyst is kept in an appropriate temperature range (see a range R in FIG. 13). That is, if exhaust gas having a temperature within the range R is supplied, the catalyst bed temperature enters the emission purification window, so that high purification rate can be achieved.
However, since the temperature of exhaust gas that flows into the emission control catalyst changes greatly depending on the state of engine operation, the exhaust gas temperature sometimes deviates from the temperature range of high purification rate. That is, as indicated in FIG. 13, the emission control catalyst experiences inflow of excessively low-temperature exhaust gas (e.g., a temperature Tc in FIG. 13), or inflow of excessively high-temperature exhaust gas (e.g., a temperature Th in FIG. 13) during real operation. Therefore, emission control is not always performed at high purification rate.
Since these drawbacks are attributed to characteristics of the emission control catalyst as mentioned above, the drawbacks can be resolved by changing the composition of the catalyst substance so as to enlarge the emission purification window. However, the changing of the catalyst composition is restricted by selection of an object to be removed. Therefore, in reality, the changing of the catalyst composition cannot achieve considerable enlargement of the emission purification window.